So I tried something else. To check if it is really the Calibre books causing the problem, I removed them all, leaving a few books I downloaded from feedbooks.
I tried using the dictionary, changing dictionary, open and close books. Everything worked as expected.
Then I tried to connect to the internet. The browser started loading the page and became unresponsive with the progress bar half-way. Back and menu buttons don't work. Finally I got an " Activity Browser (in application Browser) is not responding. Force Close or Wait. I tried clicking Force close but the unit seemed unresponsive. Then I got a black screen followed by "Pictures" where all pictures were without a thumbnail. I now deleted all custom images as well.
Is this a possibly related issue?
Now I downloaded again one Calibre created epub to see if it causes problems or not. I'll test it for a few days to see if this issue is solved or not.
Last edited by entodoays; 12-22-2012 at 12:06 PM.
Reason: added detail.